€¦ · LowTemperaturePhysicsDivision(TT) Overview Low Temperature Physics Division Fachverband Tiefe Temperaturen (TT) RudolfGross Walther-MeissnerInstitut BayerischeAkademiederWissensch

Low Temperature Physics Division (TT) Overview

Low Temperature Physics DivisionFachverband Tiefe Temperaturen (TT)

Rudolf GrossWalther-Meissner Institut

Bayerische Akademie der WissenschaftenWalther-Meissner Str. 8

85748 [email protected]

Overview of Invited Talks and Sessions(lecture rooms H18, H19, H20, and H21; poster A and D1)

Symposia co-organized by TT:

SYEL Energy Landscapes: Statistical Physics of (Spin-)Glasses, Biomolecules, Clusters andOptimization ProblemsOrganisation: Michael Kastner (Stellenbosch), Andreas Heuer(Universität Münster), Martin Weigel (Universität Mainz),Alexander Hartmann (Universität Oldenburg).Joint symposium of the divisions DY, CPP, DF, TT — see SYEL for the full program of the Symposium

SYEL 1.1 Mon 10:00–10:30 H1 Energy Landscapes of clusters, glasses, and biomolecules — ∙DavidWales

SYEL 1.2 Mon 10:30–11:00 H1 Order parameters and energy landscapes for protein folding and mis-folding — ∙Steven Plotkin

SYEL 1.3 Mon 11:00–11:30 H1 Nuclear Spins Reveal the Microscopic Nature of Tunneling Systems inGlasses — ∙Christian Enss

SYEL 1.4 Mon 11:30–12:00 H1 Energy landscapes and phase transitions — ∙Lapo CasettiSYEL 1.5 Mon 12:00–12:30 H1 Phase transitions in spin glasses — ∙Peter YoungSYEL 1.6 Mon 12:30–13:00 H1 Statistical physics of inverse problems — ∙Riccardo Zecchina

SYGN Spin-Orbit Coupling and Spin Relaxation in Graphene and Carbon NanotubesOrganization: Milena Grifoni (Universität Regensburg), Markus Morgenstern (RWTH Aachen), Jürgen Smet (MPI-FKF,Stuttgart).Joint symposium of the divisions TT, MA, HL, DS, MM — see SYNG for the full program of the Symposium

SYGN 1.1 Mon 14:00–14:35 H1 Models for spin-orbit coupling in graphene — ∙Francisco GuineaSYGN 1.2 Mon 14:35–15:10 H1 Spin-orbit coupling and spin relaxation in carbon nanotube quantum

dots — ∙Ferdinand KuemmethSYGN 1.3 Mon 15:10–15:45 H1 Spin-orbit interaction in carbon nanotubes probed in pulsed magnetic

fields — ∙Sungho JhangSYGN 1.4 Mon 16:00–16:35 H1 Wigner molecules and spin-orbit coupling in carbon-nanotube quantum

dots — ∙Massimo RontaniSYGN 1.5 Mon 16:35–17:10 H1 Spin relaxation and decoherence in graphene quantum dots — ∙Guido

BurkardSYGN 1.6 Mon 17:10–17:45 H1 Spin transport in graphene field effect transistors — ∙Bart van Wees


Focused Sessions:

TT 13 Topological Defects in Electronic Systems (Joint Session with MA)Organisation: Roderich Moessner (MPI-PKS Dresden)

TT 13.1 Tue 9:30–10:00 H20 Skyrmions in Chiral Magnets — ∙Ulrich K. RößlerTT 13.2 Tue 10:00–10:30 H20 Dirac Strings and Magnetic Monopoles in the Spin Ice, Dy2Ti2O7 —

∙David Jonathan Pryce MorrisTT 13.3 Tue 10:30–11:00 H20 Manifestations of monopole physics in spin ice materials — ∙Claudio

CastelnovoTT 13.4 Tue 11:00–11:30 H20 Skyrmion Lattices in Pure Metals and Strongly Doped Semiconductors

— ∙Christian PfleidererTT 13.5 Tue 11:45–12:15 H20 Skyrmion lattice in MnSi — ∙Achim RoschTT 13.6 Tue 12:15–12:45 H20 Topological Insulators in Applied Fields: Magnetoelectric Effects and

Exciton Condensation — ∙Joel MooreTT 13.7 Tue 12:45–13:15 H20 Probing non-Abelian statistics with quasiparticle interferometry —

∙Kirill ShtengelTT 13.8 Tue 13:15–13:45 H20 Spin Hall effects in HgTe Quantum Well Structures — ∙Laurens W.

Molenkamp

TT 21 Quantum Criticality in Strongly Correlated MetalsOrganisation: Frank Steglich (MPI-CPfS, Dresden)

TT 21.1 Wed 9:30–10:10 H20 Quantum Criticality, Kondo Breakdown, and Fermi Surfaces — ∙QimiaoSi

TT 21.2 Wed 10:10–10:50 H20 Tuning magnetic quantum phase transitions — ∙Hilbert v. LöhneysenTT 21.3 Wed 10:50–11:30 H20 Orbital-selective Mott transitions: Heavy Fermions and beyond —

∙Matthias VojtaTT 21.4 Wed 11:40–12:20 H20 Interaction of the magnetic instability and the Fermi surface reconstruc-

tion in YbRh2Si2 — ∙Sven FriedemannTT 21.5 Wed 12:20–13:00 H20 Novel electronic states near discontinous quantum phase transitions —

∙Philipp Gegenwart

TT 25 Iron-Based SuperconductorsOrganisation: Rüdiger Klingeler (IFW Dresden), Carsten Honerkamp (Universität Würzburg)

TT 25.1 Wed 14:00–14:30 H20 Fermiology of Fe-Pnictide Superconductors Revealed by Quantum Os-cillations — ∙James Analytis

TT 25.2 Wed 14:30–15:00 H20 Magnetic degeneracy and hidden metallicity of the spin density wavestate in Fe-based superconductors — ∙Ilya Eremin

TT 25.3 Wed 15:00–15:30 H20 Muon spin relaxation and Moessbauer studies of iron pnictide super-conductors — ∙Hans-Henning Klauss

TT 25.4 Wed 15:45–16:15 H20 Interplay among lattice, orbital and spin degrees of freedom in ironpnictides — ∙Roser Valenti

TT 25.5 Wed 16:15–16:45 H20 Lattice dynamics and magnetism in layered iron based superconductors— ∙Thomas Brückel

TT 25.6 Wed 17:00–17:15 H20 ARPES studies on FeAs-based superconductors and their parent com-pounds — ∙Jörg Fink

TT 25.7 Wed 17:15–17:30 H20 Energy and temperature dependence of spin fluctuations in electron-doped iron arsenide superconductors — ∙Dmytro Inosov

TT 25.8 Wed 17:30–17:45 H20 Doping evolution of the electronic density of states and the gap symme-try in Co-doped 122 iron pnictides — ∙Frédéric Hardy

TT 25.9 Wed 17:45–18:00 H20 Mössbauer high pressure and magnetic field studies of the superconduc-tor FeSe — ∙Claudia Felser


TT 28 Time-Resolved Spectroscopy in Correlated Electron Systems: Experiment and TheoryOrganisation: Marcus Kollar (Universität Augsburg), Dirk Manske (MPI-FKF, Stuttgart), Martin Wolf (FHI Berlin)

TT 28.1 Thu 9:30–10:00 H18 Angle- and time-resolved photoelectron spectroscopy of charge densitywave materials — ∙Uwe Bovensiepen

TT 28.2 Thu 10:00–10:30 H18 Many Body Theory for Time-Resolved Pump/Probe Photoemission andits Solution via Nonequilibrium Dynamical Mean-Field Theory — ∙JamesFreericks

TT 28.3 Thu 10:30–10:45 H18 Electron-phonon interaction in 122-iron pnictides investigated by fem-tosecond time-resolved ARPES. — ∙Rocío Cortés

TT 28.4 Thu 10:45–11:15 H18 Time resolved photoemission and THz spectroscopy of high temperaturesuperconductors — ∙Luca Perfetti

TT 28.5 Thu 11:30–12:00 H18 Relaxation of strongly correlated electron systems: Insights fromnonequilibrium dynamical mean-field theory — ∙Martin Eckstein

TT 28.6 Thu 12:00–12:15 H18 Quantum interference between photo-excited states in a solid-state Mottinsulator — ∙Simon Wall

TT 28.7 Thu 12:15–12:45 H18 Two-Component Dynamics of the Order Parameter of High TemperatureBi2Sr2CaCu2O8+𝛿 Superconductors Revealed by Time-Resolved RamanScattering — ∙Michael Alexander Rübhausen

TT 28.8 Thu 12:45–13:00 H18 Ultrafast non-equilibrium dynamics in conventional and unconventionalsuperconductors — ∙Andreas Schnyder

Further Invited Talks:

TT 5.1 Mon 14:00–14:30 H18 Field-Induced Berezinskii-Kosterlitz-Thouless Transition in a 2d Spin-Dimer System — ∙Michael Lang

TT 7.8 Mon 16:00–16:30 H20 Polar Kerr Effect of Unconventional Superconductors — ∙Aharon Ka-pitulnik

TT 15.4 Tue 14:45–15:15 H18 Superconductivity vs. Superinsulation in TiN Thin Films — ∙ChristophStrunk

TT 17.6 Tue 15:30–16:00 H20 Nature of Pairing in the FeAs Superconductors — ∙Siegfried GraserTT 19.8 Wed 11:15–11:45 H18 Spectroscopy on Strongly Correlated Electron Materials — ∙Liu Hao

TjengTT 23.4 Wed 14:45–15:15 H18 Superconducting Flux Qubits in Circuit QED and Detection of Weak

Microwave Signals — ∙Achim MarxTT 24.6 Wed 15:30–16:00 H19 Fermi Surface Evolution in an Electron-Doped Cuprate Superconductor

Revealed by High-Field Magnetotransport — ∙Mark KartsovnikTT 32.4 Thu 14:45–15:15 H18 Dispersion of the Excitations of Fractional Quantum Hall States —

∙Jurgen SmetTT 40.1 Fri 10:15–10:45 H20 Heating, Heat Conduction and Cooling in Molecular Junctions —

∙Abraham NitzanTT 38.4 Fri 11:00–11:30 H18 Neutron Scattering Studies of Spin-Ladders — ∙Bella Lake


Sessions:

CE: Correlated Electrons FS: Focused Session NE: NanoelectronicsSC: Superconductivity TR: Transport MLT: Matter at Low Temperature

TT 1.1–1.10 Mon 10:15–13:00 H18 TR: Graphene 1TT 2.1–2.8 Mon 10:15–12:30 H19 SC: Applications and Measuring DevicesTT 3.1–3.6 Mon 10:15–11:45 H20 TR: Fluctuations and NoiseTT 4.1–4.9 Mon 10:15–12:45 H21 CE: Quantum Impurities, Kondo PhysicsTT 5.1–5.12 Mon 14:00–17:30 H18 CE: Low-dimensional Systems - Materials 1TT 6.1–6.16 Mon 14:00–18:15 H19 TR: Nanoelectronics II: Spintronics and MagnetotransportTT 7.1–7.14 Mon 14:00–18:00 H20 SC: Heterostructures, Andreev Scattering, Proximity Effect,

CoexistenceTT 8.1–8.14 Mon 14:00–17:45 H21 CE: (General) TheoryTT 9.1–9.60 Mon 14:00–18:00 Poster A SC: Poster SessionTT 10.1–10.7 Mon 14:00–18:00 Poster A Measuring Devices, Cryotechnique: Poster SessionTT 11.1–11.12 Tue 9:30–12:45 H18 CE: Low-dimensional Systems - Materials 2TT 12.1–12.13 Tue 9:30–13:00 H19 CE: Metal-Insulator Transition 1TT 13.1–13.8 Tue 9:30–13:45 H20 FS: Topological Defects in Electronic SystemsTT 14.1–14.12 Tue 9:30–12:45 H21 TR: Graphene 2TT 15.1–15.8 Tue 14:00–16:15 H18 MLT: Quantum Liquids, Bose-Einstein Condensates, Ultra-

cold Atoms, ... 1TT 16.1–16.9 Tue 14:00–16:15 H19 CE: Quantum-Critical Phenomena 1TT 17.1–17.7 Tue 14:00–16:15 H20 SC: Iron-Based Superconductors - Theoretical ApproachesTT 18.1–18.8 Tue 14:00–16:15 H21 TR: Quantum Coherence and Quantum Information Systems

1TT 19.1–19.12 Wed 9:30–13:00 H18 CE: Metal-Insulator Transition 2TT 20.1–20.13 Wed 9:30–13:00 H19 SC: Fabrication and Characterization of Iron-Based and

Other SuperconductorsTT 21.1–21.5 Wed 9:30–13:00 H20 FS: Quantum Criticality in Strongly Correlated MetalsTT 22.1–22.14 Wed 9:30–13:15 H21 CE: Spin Systems and Itinerant MagnetsTT 23.1–23.10 Wed 14:00–17:00 H18 TR: Quantum Coherence and Quantum Information Systems

2TT 24.1–24.16 Wed 14:00–18:45 H19 SC: Properties, Electronic Structure, MechanismsTT 25.1–25.9 Wed 14:00–18:00 H20 FS: Iron-Based SuperconductorsTT 26.1–26.17 Wed 14:00–18:45 H21 CE: Heavy FermionsTT 27.1–27.84 Wed 14:00–18:00 Poster D1 CE: Poster SessionTT 28.1–28.8 Thu 9:30–13:00 H18 FS: Time-Resolved Spectroscopy in Correlated Electron Sys-

tems:Experiment and Theory

TT 29.1–29.13 Thu 9:30–13:00 H19 TR: Nanoelectronics I: Quantum Dots, Wires, Point Con-tacts 1

TT 30.1–30.13 Thu 9:30–13:00 H20 CE: Low-dimensional Systems - Models 1TT 31.1–31.13 Thu 9:30–13:00 H21 SC: Tunnelling, Josephson JunctionsTT 32.1–32.14 Thu 14:00–18:00 H18 MLT: Quantum Liquids, Bose-Einstein Condensates, Ultra-

cold Atoms, ... 2TT 33.1–33.15 Thu 14:00–18:00 H19 TR: Nanoelectronics I: Quantum Dots, Wires, Point Con-

tacts 2TT 34.1–34.13 Thu 14:00–17:30 H20 SC: Iron-Based Superconductors - 122TT 35.1–35.14 Thu 14:00–17:45 H21 TR: Nanoelectronics III: Molecular Electronics 1TT 36.1–36.34 Thu 14:00–18:00 Poster A TR: Poster SessionTT 37.1–37.31 Thu 14:00–18:00 Poster A MLT: Poster SessionTT 38.1–38.8 Fri 10:15–12:45 H18 CE: Quantum-Critical Phenomena 2TT 39.1–39.9 Fri 10:15–12:45 H19 SC: Iron-Based Superconductors - 1111TT 40.1–40.9 Fri 10:15–13:45 H20 TR: Nanoelectronics III: Molecular Electronics 2TT 41.1–41.8 Fri 10:15–12:30 H21 CE: Low-dimensional Systems - Models 2

Annual General Meeting of the Low Temperature Physics Division

Thursday 18:30-20:00 H19

Low Temperature Physics Division (TT) Monday

TT 1: TR: Graphene 1

Time: Monday 10:15–13:00 Location: H18

TT 1.1 Mon 10:15 H18Orbitally controlled Kondo effect of Co ad-atoms ongraphene — ∙Tim Wehling1, Alexander Balatsky2, MikhailKatsnelson3, Alexander Lichtenstein1, and Achim Rosch4— 1I. Institut für Theoretische Physik, Universität Hamburg,Jungiusstrasse 9, 20355 Hamburg, Germany — 2Theoretical Division,Los Alamos National Laboratory, Los Alamos, New Mexico 87545,USA — 3Institute for Molecules and Materials, Radboud University ofNijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands— 4Institute for Theoretical Physics, University of Cologne, 50937Cologne, GermanyBased on ab-initio calculations we identify possible scenarios for theKondo effect due to Co ad-atoms on graphene. For a Co atom absorbedon top of a carbon atom, the Kondo effect is quenched by spin-orbitcoupling below an energy scale of ∼15 K. For Co with spin 𝑆 = 1/2located in the center of a hexagon, an SU(4) Kondo model describesthe entanglement of orbital moment and spin at higher energies, whilebelow ∼ 60 meV spin-orbit coupling leads to a more conventional SU(2)Kondo effect. The interplay of the orbital Co physics and the pecu-liar band-structure of graphene is directly accessible in Fourier trans-form tunneling spectroscopy or in the gate-voltage dependence of theKondo temperature displaying a very strong, characteristic particle-hole asymmetry.

TT 1.2 Mon 10:30 H18Revivals of quantum wave packets in graphene — ∙ViktorKrückl1 and Tobias Kramer1,2 — 1Institut für TheoretischePhysik, Universität Regensburg, 93040 Regensburg — 2Departmentof Physics, Harvard University, Cambridge, MA 02138, USAWe investigate the propagation of wave-packets on graphene in a per-pendicular magnetic field. The wave-packet evolution in graphenediffers drastically from the one in an electron gas and shows a richrevival structure similar to the dynamics of highly excited Rydbergstates [1]. We present a detailed analysis of the occurring collapses,revivals and fractional revivals analytically as well as numerically. Inorder to study the impact of disorder on the effect we apply our novelnumerical scheme to solve the wave-packet propagation on the effec-tive single-particle Dirac-Hamiltonian of graphene in the presence ofrandom impurity potentials.[1] Viktor Krueckl and Tobias Kramer, New J. Phys. 11 093010 (2009)

TT 1.3 Mon 10:45 H18Externally induced spin relaxation in graphene — ∙JanBundesmann1, Michael Wimmer1,2, and Klaus Richter1 —1Institut für theoretische Physik, Universität Regensburg, 93040 Re-gensburg, Germany — 2Instituut-Lorentz, Universiteit Leiden, 2300RA Leiden, The NetherlandsIn the fast growing research field of spintronics graphene seems to bea promising candidate. From theory weak spin-orbit interaction is ex-pected in pure carbon-based materials. However, experimental resultsand theoretical predictions differ by several orders of magnitude: spinlifetimes in the experiment are much smaller than, e. g., the ones ob-tained from recent DFT calculations [1].In our calculations we will include also externally induced spin-orbitinteractions. Sources for this might be impurities in the substrate oradsorbed atoms. For this investigation we set up a tight-binding modelfor graphene including intrinsic and Rashba-type spin-orbit interac-tions. By local variation of the Rashba parameter we model systemswith the above introduced sources of spin-orbit interaction and studyspin-orbit effects on quantum transport.

[1] M. Gmitra, S. Konschuh, C. Ertler, C. Ambrosch-Draxl and J.Fabian; arxiv:0904.3315

TT 1.4 Mon 11:00 H18Coulomb interaction in graphene: Relaxation rates andtransport — ∙Michael Schütt1, Pavel Ostrovsky2, IgorGornyi2, and Alexander Mirlin1,2 — 1Institut für Theorie derKondensierten Materie, Karlsruhe Institute of Technology (KIT),76128 Karlsruhe, Germany — 2Institut für Nanotechnologie, KarlsruheInstitute of Technology (KIT), 76021 Karlsruhe, GermanyWe study electron transport in graphene with Coulomb interaction atfinite temperatures by using Keldysh diagrammatics. In the case of

clean graphene we obtain the total scattering rate, the transport scat-tering rate, and the energy relaxation rate at the Dirac point. Since thetotal scattering rate diverges graphene exhibits a non-Fermi-liquid be-havior similar to disordered metals. Unlike metals clean graphene hasa finite conductivity due to the Coulomb interaction. For conductivitywe obtain the same analytic behavior as was found using the Boltz-mann approach [1,2]. We analyze the plasmon spectrum of grapheneand formulate quantum kinetic equations to describe transport in thecrossover between the Coulomb interaction dominated regime and thedisorder dominated regime.

[1] L. Fritz et al., Phys. Rev. B 78:085416 (2008).[2] A. Kashuba, Phys. Rev. B 78:085415 (2008).

TT 1.5 Mon 11:15 H18Is it possible to detect edge states in graphene quantum dots?— ∙Michael Wimmer1, Anton R. Akhmerov1, and FranciscoGuinea2 — 1Instituut-Lorentz, Universiteit Leiden, The Netherlands— 2Instituto de Ciencia de Materiales de Madrid, SpainWe analyze the single particle states at the edges of graphene quan-tum dots of arbitrary shapes. By combining analytical and numericalarguments, we show that localized edge states, distinct from extendedones, exist in dots of all dimensions. The number of these states isproportional to the circumference of the dot measured in lattice con-stants. Perturbations breaking electron-hole symmetry shift the edgestates away from zero energy but do not change their total amount.

15 min. break

TT 1.6 Mon 11:45 H18Graphene: Relativistic transport in a nearly perfect quantumliquid — ∙Lars Fritz1, Markus Mueller2, Joerg Schmalian3,and Subir Sachdev4 — 1Universitaet zu Koeln, Institut fuer theo-retische Physik, Zuelpicher Strasse 77, 50937 Koeln — 2The AbdusSalam International Centre for Theoretical Physics, Strada Costiera11, 34151 Trieste, Italy — 3Department of Physics and AstronomyIowa State University Ames, Iowa 50011, USA — 4Harvard Univer-sity, 17 Oxford Street, Cambridge, MA 02138, USAElectrons and holes in clean, charge-neutral graphene behave like astrongly coupled relativistic liquid. The thermo-electric transportproperties of the interacting Dirac quasiparticles are rather special, be-ing constrained by an emergent Lorentz covariance at hydrodynamicfrequency scales. At small carrier density and high temperatures,graphene exhibits signatures of a quantum critical system with an in-elastic scattering rate set only by temperature, a conductivity with anearly universal value, solely due to electron-hole friction, and a verylow viscosity. In this regime one finds pronounced deviations fromstandard Fermi liquid behavior. These results, obtained by Boltz-mann transport theory at weak electron-electron coupling, are fullyconsistent with the predictions of relativistic hydrodynamics.

TT 1.7 Mon 12:00 H18Hyperfine interaction and electron-spin decoherencein graphene and carbon nanotube quantum dots —∙Jan Fischer1, Bjoern Trauzettel2, and Daniel Loss1 —1Department of Physics, University of Basel, Klingelbergstrasse 82,4056 Basel, Switzerland — 2Institute of Theoretical Physics and As-trophysics, University of Würzburg, D-97074 Würzburg, GermanyWe analytically calculate the nuclear-spin interactions of a single elec-tron confined to a carbon nanotube or graphene quantum dot [1].While the conduction-band states in graphene are p-type, the accor-dant states in a carbon nanotube are sp-hybridized due to curvature.This leads to an interesting interplay between isotropic and anisotropichyperfine interactions. By using only analytical methods, we are ableto show how the interaction strength depends on important physi-cal parameters, such as curvature and isotope abundances. We showthat for the investigated carbon structures, the 13C hyperfine couplingstrength is less than 1 𝜇eV, and that the associated electron-spin de-coherence time can be expected to be several tens of microseconds orlonger, depending on the abundance of spin-carrying 13C nuclei. Fur-thermore, we find that the hyperfine-induced Knight shift is highlyanisotropic, both in graphene and in nanotubes of arbitrary chirality.

[1] J. Fischer, B. Trauzettel, D. Loss, Phys. Rev. B 80, 155401


(2009)

TT 1.8 Mon 12:15 H18Spin transport in graphene with inhomogeneous spin-orbitcoupling — ∙Dario Bercioux1 and Alessandro De Martino2 —1Freiburg Institute for Advanced Studies, Albert-Ludwigs-Universität,D-79104 Freiburg, Germany — 2Institut für Theoretische Physik, Uni-versität zu Köln, Zülpicher Straße 77, D-50937 Köln, GermanyRecent theoretical [1] and experimental [2] works have shown thatspin-orbit couplings in graphene can play a relevant role. Motivatedby these results, we address the problem of spin transport in graphenethrough spin-orbit nanostructures, i.e. regions of inhomogeneous spin-orbit coupling on the nanometer scale. In analogy with the case ofusual two-dimensional electron gases, we discuss the phenomenon ofspin-double refraction [3,4] and its consequences on the spin polariza-tion. In particular we study the transmission properties of a single-and a double-interface between a normal region and a region with finitespin-orbit coupling, and analyze the polarization properties of thesesystems. In addition, for the case of the single interface, we considerthe formation of bound states localized at the interface, analogous tothe states occuring at the edges of graphene in the weak topologicalinsulator regime discussed by Kane and Mele [5].[1] D. Huertas-Hernando, et al., Phys. Rev. Lett. 103, 146801 (2009).[2] A. Varykhalov, et al., Phys. Rev. Lett. 101, 157601 (2008).[3] V. M. Ramaglia, et al., Eur. Phys. J. B 36, 365 (2003).[4] V. M. Ramaglia, et al., J. Phys.: Condens. Matter 16, 9143 (2004).[5] C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005).

TT 1.9 Mon 12:30 H18Edge effects in quantum transport and quasiparticle spec-tra of graphene nanostructures — ∙Jürgen Wurm1,2, KlausRichter1, and Inanç Adagideli2 — 1Institut für TheoretischePhysik, Universität Regensburg, 93040 Regensburg — 2Faculty of En-gineering and Natural Sciences, Sabancı University, Orhanlı - Tuzla,34956, Turkey

In this work, we focus on the spectral and transport properties ofgraphene nanostructures. In recent work, we studied the effects ofedges on the transport and spectral properties of graphene quantumdots, as well as on the conductance of graphene nanoribbons numer-ically [1,2]. Some edges can lead to effective time reversal symmetrybreaking, others are effective intervalley scatterers. In this work, wedevelop a theory that is capable of handling such effects in graphenenanostructures. We do this in two steps. First, we derive an exact ex-pression for the Green function of a graphene flake, where each term inthis expansion corresponds to the specific number of times the quasi-particle hits the edge. Second, we use the Green function to calculate:(i) the spectra for closed systems and (ii) the conductance of opensystems. In particular, we focus on phase coherent effects, such asthe weak localization correction to the average conductance, and theuniversal conductance fluctuations. Moreover, we show how the sizeof these effects depends on the edges.

[1] J. Wurm et al., Phys. Rev. Lett. 102, 056806 (2009)[2] J. Wurm et al., New J. Phys. 11, 095022 (2009)

TT 1.10 Mon 12:45 H18Charge transport in disordered superconductor-graphenejunctions — ∙Georgo Metalidis1, Dmitry Golubev2, and GerdSchön1 — 1Institut für Theoretische Festkörperphysik, Karlsruher In-stitut für Technologie, D-76131 Karlsruhe, Germany — 2Institut fürNanotechnologie, Karlsruher Institut für Technologie, D-76021 Karl-sruhe, GermanyWe consider the charge transport through superconductor-graphenetunnel junctions, including the effect of disorder. Coherent scatteringon elastic impurities in the graphene layer can give rise to multiplereflections at the graphene-superconductor interface, and can therebyincrease the probability of Andreev reflection, leading to an enhance-ment of the subgap conductance above its classical value. Althoughthe phenomenon is known already from heterostructures involving nor-mal metals, we have studied how graphenes peculiar dispersion relationinfluences the effect.

TT 2: SC: Applications and Measuring Devices


TT 2.1 Mon 10:15 H19Josephson-Cantilever with integrated THz antenna —∙Christian Brendel, Heiko Neeland, Tobias Vosskoetter, JanM. Scholtyssek, and Meinhard Schilling — Institut für Elek-trische Messtechnik und Grundlagen der Elektrotechnik, TechnischeUniversität Braunschweig, Hans-Sommer-Str. 66, D-38106 Braun-schweig, GermanyWe imaged the three dimensional microwave power distributions atthe open end of multimode circular waveguides and in the free spacein the THz range. The measurement setup consists of a grating-tunedCO2 laser (emission 9 - 11 𝜇m) to pump a FIR laser with an out-put frequency range from 584GHz up to 4.2THz. As scanning sensorwe employ a Josephson junction from the high-temperature supercon-ductor YBa2Cu3O7 on a vibrating cantilever prepared from a LaAlO3-bicrystal. The setup is mounted inside a vacuum chamber on x-, y- andz-tables with submicrometer resolution and is cooled to a temperatureof about 50K by a cryocooler. We realized a quasi-optic THz-lens sys-tem and determined the beam parameters at the Josephson-cantileverposition. The small change of the differential resistance in the Shapirostep is proportional to the microwave power. To improve the signalto noise ratio in the measured curent voltage curve we developed newJosephson cantilever designs with different THz antenna strutures.

We wish to acknowledge the financial support of C. Brendel by theBraunschweig International Graduate School of Metrology.

TT 2.2 Mon 10:30 H19Metallic magnetic calorimeters for high-resolution x-ray spec-troscopy — ∙Jan-Patrick Porst, Sebastian Kempf, AndreaKirsch, Andreas Pabinger, Christian Pies, Philipp Ranitzsch,Sönke Schäfer, Falk von Seggern, Loredana Gastaldo, An-dreas Fleischmann, and Christian Enss — Kirchhoff-Institut fürPhysik, Universität Heidelberg, INF 227, 69120 HeidelbergMetallic magnetic calorimeters (MMC) are calorimetric particle detec-tors, typically operated at temperatures below 100 mK, that make use

of a paramagnetic temperature sensor to transform the temperaturerise upon the absorption of a particle in the detector into a measurablechange of magnetic flux in a dc-SQUID. During the last years we havestarted to develop MMCs for a wide variety of applications, rangingfrom beta- and gamma-spectrometry over the spatially resolved de-tection of accelerated molecule fragments to arrays of high resolutionx-ray detectors. For x-ray energies up to 6 keV an energy resolution of2.7 eV (FWHM) has been demonstrated and we expect that this canbe pushed below 1 eV with the next generation of devices. We summa-rize the physics of MMCs and the presently used readout schemes aswell as the typically observed noise contributions and their impact onthe energy resolution. We discuss general design considerations, themicro-fabrication and the performance of micro-fabricated devices. Inthis field large progress has been achieved in the last years and thethermodynamic properties of most materials approach bulk values al-lowing for optimal and predictable performance.

TT 2.3 Mon 10:45 H19Moving a vortex by the tip of a magnetic force microscope —∙Ernst Helmut Brandt — Max-Planck-Instiut für MetallforschungIn a recent paper [1] a magnetic force microscope (MFM) was employedto image and manipulate individual vortices in a thick single crystalof YBa2Cu3O7. When the magnetic tip performed a zig-zag motion,wiggling fast along 𝑥 and moving slowly along 𝑦, a large enhancementof the excursion of the vortex end at the upper surface (in the crys-talline a-b plane) was observed along 𝑦, the vortex path covered anelliptical area with axes ratio max(𝑦)/max(𝑥) ≫ 1. As a first steptowards a more detailed theory, we consider the vortex as an elasticstring which is uniformly pinned by point defects and is driven by themagnetic force exerted on the vortex near the surface by the tip ofthe MFM. The tip is approximated by a magnetic monopole and theanisotropy of YBa2Cu3O7 in the a-b plane is accounted for. When thetip moves with wiggles, the vortex is curved and twisted, its motionpenetrating to a maximum depth 𝑧0 below which the vortex remainsrigidly pinned in its straight initial position. Our theory [2] reproduces


the path of the vortex end observed in the experiments [1].[1] O. M. Auslaender et al., Nature Physics 5, 35 (2009).[2] E. H. Brandt, G. P. Mikitik and E. Zeldov, Phys. Rev. B 80,

054513, 1-10 (2009).

TT 2.4 Mon 11:00 H19Barriers for vortex avalanche propagation in MgB2 thin films— ∙Sebastian Treiber1 and Joachim Albrecht2 — 1Max-Planck-Institut für Metallforschung, Stuttgart, Germany — 2HochschuleAalen, GermanAt temperatures below 10K the critical state in MgB2 thin films getsunstable. This leads to chaotic motion of magnetic vortices and den-dritic flux density patterns. Since the critical current is zero inside thedendrites this effect leads to a strong suppression of possible transportcurrents.

We prepared MgB2 films containing areas of different current den-sities and thermal conductivities, respectively. Magneto-optical inves-tigations revealed that the propagation of vortex avalanches can bemanipulated by this local variations. It is found that boundaries toareas of higher current density or enhanced thermal conductivity canact like a barrier for the dendrites. The investigations also show thatthe critical current density around the dendrites is not constant whichcan not be described by classical critical state models.

TT 2.5 Mon 11:15 H19Energy relaxation processes in YBCO thin films studied byfrequency and time-domain techniques — ∙Petra Probst1,Dagmar Rall1,2, Matthias Hofherr1, Stefan Wünsch1, Kon-stantin Ilin1, and Michael Siegel1 — 1Institut für Mikro- and Na-noelektronische Systeme, Karlsruher Institute of Technology, Hertzs-trasse 16, 76187 Karlsruhe, Germany — 2Lichttechnisches Institut,Karlsruher Institute of Technology, Engesserstrasse 13, 76131 Karl-sruhe, GermanyThe development of ultra-fast detectors with time resolutions in thepicosecond range requires the analysis and understanding of the dy-namics in the energy relaxation processes in thin films. A systematicstudy of the energy relaxation processes in YBCO thin films on sap-phire substrate has been performed. Pulsed-laser deposited YBCOsamples between 20 and 60 nm film thickness were fabricated andcharacterized by means of frequency and time domain techniques byexcitation of the samples with optical radiation. The characteristic en-ergy relaxation time was extracted from the thin film sample responseaccording to the two-temperature model. The extracted time constantsof the two techniques showed good agreement. We have observed anincrease of the energy relaxation time from 500 ps to 4 ns with increaseof the film thickness from 20 to 60 nm, respectively. The obtained de-pendence of the characteristic time on film thickness we attribute tothe escape of non-equilibrium phonons from the YBCO films into sub-strate. Details of the experimental methods and results on the energyrelaxation in thin YBCO films and the applied theoretical model willbe discussed.

15 min. break

TT 2.6 Mon 11:45 H19AC loss data for YBCO double-pancake coils — ChristianStiehler1, ∙Vadim Grinenko1, Konstantin Nenkov1, MichalVojenciak2, Günter Fuchs1, Bernhard Holzapfel1, and Lud-wig Schultz1 — 1Leibniz Institute for Solid State and Materi-als Research Dresden, Helmholtzstraße 20, 01067 Dresden, Germany— 2Institute of Electrical Engineering, Slovak Academy of Sciences,Dubravsk’a cesta 9, 841 04 Bratislava, Slovak Republic

Since the second generation of superconducting tapes, YBCO coatedconductors, are commercially available, large scale AC applicationsseem to be very promising provided that AC loss of the 2G tapes canbe reduced. In this work, critical current measurements as well as ACloss experiments of YBCO double-pancake coils and 2G tapes wereperformed. An analysis of the magnetic field distribution at the coil’sexterior, especially in critical points, shows no self-field limitation ofthe critical current density. Magnetisation loss data were obtained forYBCO double-pancake coils by applying magnetic AC fields at 77 K.It is demonstrated that the AC loss can be reduced by means of mag-netic shielding. Furthermore, AC transport losses have been measuredat temperatures between 65 and 77 K for frequencies between 36 and288 Hz. The AC loss data were found to be dominated by hysteresislosses. The obtained results are compared with AC loss data reportedfor YBCO pancake coils.

TT 2.7 Mon 12:00 H19Development of a micro-Hall magnetometer for the studyof light-induced effects in magnetic molecules — ∙RobertLusche, Jan Dreiser, and Oliver Waldmann — PhysikalischesInstitut, Universität Freiburg, D-79104 Freiburg, GermanyControlling magnetic properties in magnetic molecules with light is anintriguing effect and has recently attracted renewed interest [1]. Inorder to measure the magnetization in the presence of light we rely onmicro-Hall-senors in our setup. Micro-Hall probes have been provento be a simple and robust tool for magnetic measurements over a largerange of experimental conditions. Here we present the design of ourhome-built micro-Hall magnetometer including an irradiation unit. Itcan be operated at temperatures down to 1.4K and with magneticfields of up to 5.5T, and provides a resolution better than 10−7 emu.We demonstrate and characterize the performance by measurementson known spin-crossover complexes, and present first measurements onnovel photomagnetic complexes.

[1] A. Bleuzen et al., Inorg. Chem. 48, 3453 (2009).

TT 2.8 Mon 12:15 H19Coherent broadband continuous-wave THz spectrometry:A powerful tool for low-energy solid-state spectroscopy atlow temperature and high magnetic field — ∙KomalavalliThirunavukkuarasu1, Holger Schmitz1, Axel Roggenbuck2,Andreas Janssen1, Anselm Deninger2, Iván Cámara Mayorga3,Joachim Hemberger1, Rolf Güsten3, and Markus Grüninger1— 1II. Physikalisches Institut, Universität zu Köln, D-50937 Köln, Ger-many — 2TOPTICA Photonics AG, Lochhamer Schlag 19, D-82166Gräfelfing, Germany — 3Max-Planck Institute for Radio Astronomy,Auf dem Hügel 69, D-53121 Bonn, GermanyWe present the development of a continuous-wave THz spectrometerand its application at low temperatures and high magnetic fields. Thespectrometer employs photomixing of two NIR-DFB diode lasers forgeneration and phase sensitive detection of THz radiation of frequencyfrom 60 GHz to 1.8 THz. A phase modulation technique and photocur-rent correction are used to accurately determine amplitude and phaseat a given frequency, and to correct for instabilities, respectively. Thecomplex optical functions can be evaluated from the full THz phase in-formation, and a very high spectral resolution in the MHz range can beachieved. Furthermore, this compact spectrometer can be integratedwithin a magnetic cryostat eliminating the need for optical windows.In this way, the investigations at high magnetic fields up to 16 T andlow temperatures up to 2 K can be achieved without loss of inten-sity. Thus, a new door is opened for exploring low-energy electronicexcitations of novel materials, lying in the sub-phonon energy regime.

TT 3: TR: Fluctuations and Noise


TT 3.1 Mon 10:15 H20Interactions, Coherence, and Multistability in TransportStatistics of coupled Quantum Dots — Gernot Schaller,∙Gerold Kiesslich, and Tobias Brandes — Institut für Theoretis-che Physik, Technische Universität Berlin, Hardenbergstr. 36, 10623BerlinWe present a novel generalized coarse-graining n-resolved master equa-

tion technique [1]. By introducing a virtual detector counting the num-ber of transferred electrons in single-electron transport the coherencesand the Lamb shift can be conveniently included. For illustration, weconsider transport through interacting levels that are either serially orparallely coupled to two leads. We show for the parallel setup thatthe coherences can lead to strong current suppression, giant Fano fac-tors and bistable transport statistics. In the serial case a finite coarsegraining time can resolve the shortcomings of the Born-Markov-secular


approximation such as unphysical currents.[1] G. Schaller, G. Kießlich, and T. Brandes, in press, Phys. Rev.

B, arXiv:0908.3620

TT 3.2 Mon 10:30 H20Spin-induced charge correlations in transport through inter-acting quantum dots with ferromagnetic leads — ∙StephanLindebaum, Daniel Urban und Jürgen König — Theoretische Phy-sik, Universität Duisburg-Essen and CeNIDE, 47048 Duisburg, Germa-nyWe study the full counting statistics of electronic transport through asingle-level quantum dot weakly coupled to two leads, with either oneor both of them being ferromagnetic[1]. Starting from a generalizedmaster equation we use a diagrammatic real-time theory to calculatethe cumulant generating function to first order in the tunnel-couplingstrength. In both considered systems, we find that the interplay of Cou-lomb interaction and finite spin polarization implies spin-correlationinduced charge correlations that give rise to super-Poissonian trans-port behavior. In the case of two ferromagnetic leads, we analyze thenon-trivial dependence of the cumulants on the angle between the non-collinear polarization directions of the leads. We find even divergingsecond and higher cumulants for spin polarizations approaching unity.But already the system with one ferromagnetic and one normal leaddisplays super-Poissonian behavior and, in addition, positive cross cor-relations between the current fluctuations of the two spin species, if theelectrons are injected from the normal electrode.

[1] S. Lindebaum, D. Urban, and J. König, Phys. Rev. B 79, 245303(2009).

TT 3.3 Mon 10:45 H20Calculating Shot Noise based on Numerical Time Evolu-tion of Transport States — ∙Alexander Branschädel1 andPeter Schmitteckert2 — 1Institut für Theorie der KondensiertenMaterie, Karlsruher Institut für Technologie, Karlsruhe, Deutschland— 2Institut für Nanotechnologie, Karlsruher Institut für Technologie,Karlsruhe, DeutschlandA method to calculate shot noise in one-dimensional systems basedon the time evolution of transport states using numerical simulationtechniques is presented. We consider the single resonant level model,consisting of a single impurity attached to non-interacting leads, withspinless fermions, where we include nearest neighbour interaction be-tween the impurity site and the leads. The time evolution is obtainedeither using exact diagonalisation for the non-interacting case or bymeans of time-dependent Density Matrix Renormalisation Group (td-DMRG) algorithms. We present results for the shot noise for finitebias voltage in the low frequency limit as well as the full frequencydependency and compare with analytical calculations.

TT 3.4 Mon 11:00 H20Quasiprobability and weak measurement of current noise ina quantum point contact — ∙Adam Bednorz1,2 and WolfgangBelzig1 — 1Fachbereich Physik, Universität Konstanz, 78457 Kon-stanz, Deutschland — 2Institute of Theoretical Physics, University ofWarsaw, Hoza 69, 00-681 Warsaw, PolandThe charge flow (counting statistics) through mesoscopic junctions iswell described by Bernoulli statistics in the long time (low frequency)limit [1] which follows from a projective detection model. The prob-lem becomes more complicated and interesting, when considering themeasurement of noncommuting observables, e.g. current and phase ortime-resolved current [2]. The latter case can be resolved in terms of

the weak measurement [3]. The idea is similar to the concept of weakvalues [4]. We show that the outcome of the weak measurement can beinterpreted in terms of a quasiprobability. Namely, the total probabil-ity distribution of the measured values of observables is a convolutionof a large, white, Gaussian detection noise and a quasiprobability (in-dependent of the detector). We show that the quasiprobability cantake negative values. The negative quasiprobability can be measured,if the Gaussian noise is subtracted, by a measurement of the fourthcumulant at high frequencies.[1] G.B. Lesovik and L.S. Levitov, Phys. Rev. Lett. 72, 538 (1994).[2] W. Belzig and Y.V. Nazarov, Phys. Rev. Lett. 87, 197006 (2001).[3] A. Bednorz and W. Belzig, Phys. Rev. Lett. 101, 206803 (2008).[4] Y. Aharonov, D.Z. Albert and L. Vaidman, Phys. Rev. Lett. 60,1351 (1988).

TT 3.5 Mon 11:15 H20Noise conductance of carbon nanotube transistors — JulienChaste1, Emiliano Pallecchi1, Pascal Morfin1, GwendalFève1, Takis Kontos1, Jean-Marc Berroir1, Pertti Hakonen2,and ∙Bernard Plaçais1 — 1Ecole Normale Superieure, LaboratoirePierre Aigrain, 24 rue Lhomond 75005 Paris, France — 2Helsinki Uni-versity of Technology, Low Temperature laboratory, Espoo, FinlandThe presentation deals with radio-frequency noise and transmissionmeasurements of high-gain single wall carbon nanotube transistors atcryogenic temperatures [1]. The gate capacitance, drain conductance,transconductance and current-noise are analyzed by relying on a bal-listic 1-dimensional scattering model whose parameter is the channelquantum capacitance that controls gate coupling. At 4 Kelvin, cur-rent and noise are thermally activated. The bias-dependent electronictemperature can be measured from the gate voltage dependence oftransconductance. A ”noise conductance” can be then deduced whichis found to obey a simple law as function of drain conductance andtransconductance as predicted by the 1D model. Finally we estimatethe charge resolution of nanotube devices for applications as fast single-shot electron detectors.

[1] J. Chaste, E. Pallecchi, P. Morfin, G. Fève, T. Kontos, J.-M.Berroir, P. Hakonen, B. Plaçais, submitted (2009).

TT 3.6 Mon 11:30 H20Inelastic noise spectroscopy in molecular junctions with mul-tiple electronic states — ∙Federica Haupt1, Tomas Novotny2,and Wolfgang Belzig1 — 1Department of Physics, Konstanz Uni-versity, Konstanz, Germany — 2Department of Condensed MatterPhysics, Charles University, Prague, Czech RepublicInelastic transport spectroscopy and shot noise measurements are es-sential investigation tools in the field of molecular electronics. In fact,while the first allows to extract information on the presence and theorientation of a molecule in a junction, the latter can be used to analyzeits individual conductance channels. So far noise measurements havebeen limited to the elastic transport regime, but more information isexpected to be provided by inelastic noise spectroscopy.

In this work we investigate the effects due to inelastic phonon scat-tering on the current noise in molecular junctions with multiple elec-tronic states. This case is particularly interesting because electron-phonon interaction may results in an effective coupling of differentelectronic states. Using the extended Keldysh-Green’s function for-malism we derive a general expression for the zero frequency noise inthe case of weak electron-phonon coupling. We compare this result tothe case in which transport is dominated by a single electronic state.Finally, we apply our theory to an experimentally relevant set-up.

TT 4: CE: Quantum Impurities, Kondo Physics


TT 4.1 Mon 10:15 H21Kondo physics in chaotic and regular mesoscopic systems —∙Rainer Bedrich1, Sebastien Burdin2, and Martina Hentschel1— 1Max Planck Institut für Physik komplexer Systeme, NöthnitzerStr. 38, 01187 Dresden, Germany — 2Condensed Matter TheoryGroup, CPMOH, UMR 5798, Université de Bordeaux I, 33405 Tal-ence, FranceWe study the mesoscopic Kondo box, consisting of a quantum spin 1/2interacting with an electronic bath as can be realized by a magnetic

impurity coupled to electrons on a quantum dot using a mean-fieldapproach for the Kondo interaction. Its numerical efficiency allowsus to analyze the Kondo temperature, the local magnetic susceptibil-ity, and the conductance statistics for a large number of samples as afunction of the temperature, the Kondo coupling, and the number ofelectrons on the dot. Here, in contrast to the Kondo effect occurringin a bulk material, the metallic host is characterized by a finite meanlevel spacing, generating deviations from the universal behavior. Wecontrast the behavior of chaotic systems, described within the randommatrix theory, to that of regular systems, e.g. rectangular quantum


dots. Besides the system geometry we find that the boundary of thedot affects the physical properties in a significant way.

TT 4.2 Mon 10:30 H21Lattice density-functional theory of the single-impurity An-derson model — ∙Waldemar Töws and Gustavo Pastor — In-stitut für Theoretische Physik, Universität Kassel, GermanyA lattice density-functional theory of the single-impurity Andersonmodel is presented. In this approach the basic variable is the single-particle density matrix 𝛾𝑖𝑗𝜎 with respect to the lattice sites. Thecentral interaction-energy functional 𝑊 [𝛾] is shown to be invariant un-der unitary transformations of orbitals in the conduction band. Thisproperty is exploited to find a unitary transformation such that thelocalized impurity orbital experiences charge fluctuations only to aparticular single-particle state of the conduction band. A simple an-alytical approximation to 𝑊 [𝛾] is then derived from the solution ofthe resulting two-level problem. This so-called two-level approxima-tion can be shown to be exact in the limit of a totally degeneratedconduction band as well as in the limit of widely separated discreteconduction-band levels. The minimization of the total energy func-tional 𝐸 [𝛾] with respect to 𝛾𝑖𝑗𝜎 yields the ground-state propertiessuch as the kinetic, interaction and total energy, as well as the occu-pation and spin polarization of the impurity orbital. The results ob-tained with the two-level-approximation for finite rings having 𝑁 ≤ 12sites are in agreement with exact Lanczos diagonalizations in all in-teraction regimes, from weak to strong correlations. In particular thesinglet-triplet gap, which determines the Kondo temperature, is cor-rectly described. This constitutes a remarkable qualitative improve-ment over mean-field approximations. Advantages and limitations ofthis approach are discussed.

TT 4.3 Mon 10:45 H21A weak coupling CTQMC study of the single impurityand periodic Anderson models with s-wave superconductingbaths — ∙David J. Luitz and Fakher F. Assaad — Institut fürtheoretische Physik und Astrophysik, Universität Würzburg, Deutsch-landWe apply the unbiased weak-coupling continuous time quantum MonteCarlo (CTQMC) method to review the physics of a single magnetic im-purity coupled to s-wave superconducting leads described by the BCSreduced Hamiltonian. As a function of the superconducting gap Δ, westudy the first order transition between the singlet and doublet (localmoment) states by examining the crossing of the Andreev bound statesin the single particle spectral function. Within DMFT, this impurityproblem provides a link to the periodic Anderson model with supercon-ducting conduction electrons (BCS-PAM). The first order transitionobserved in the impurity model is reproduced in the BCS-PAM and issignalized by the crossing of the low energy excitations in the local den-sity of states. The momentum resolved single particle spectral functionin the singlet state reveals the coherent, Bloch-like, superposition ofAndreev bound states. In the doublet or local moment phase the singleparticle spectral function is characterized by incoherent quasiparticleexcitations.

TT 4.4 Mon 11:00 H21Spectral function of the Anderson impurity model at finitetemperatures — ∙Aldo Isidori1, Hermann Freire2, and PeterKopietz1 — 1Institut für Theoretische Physik, Universität Frankfurt,Max-von-Laue Strasse 1, 60438 Frankfurt, Germany — 2Instituto deFísica, Universidade Federal de Goiás, 74.001-970, Goiânia-GO, BrasilUsing the functional renormalization group (FRG) and the numericalrenormalization group (NRG), we calculate the spectral function of theAnderson impurity model at zero and finite temperatures. In our FRGscheme spin fluctuations are treated non-perturbatively via a suitableHubbard-Stratonovich field, but vertex corrections are neglected. Al-though at zero temperature this FRG scheme does not quantitativelyreproduce the known exponential narrowing of the Kondo resonancefor large values of the interaction, a comparison with the numericallyexact NRG results shows that the FRG gives a reasonable descriptionof the spectral line-shape both at high energies and for temperatureslarger than the Kondo scale.

15 min. break

TT 4.5 Mon 11:30 H21The Kondo exciton: a quantum quench towards strong spin-

reservoir correlations — Hakan E. Türeci1, ∙Markus Hanl2,Martin Claassen1, Andreas Weichselbaum2, Theresa Hecht2,Bernd Braunecker3, Alexander Govorov4, Leonid Glazman5,Jan von Delft2, and Atac Imamoglu1 — 1ETH-Zürich — 2Ludwig-Maximilians-Universität München — 3University of Basel — 4OhioUniversity — 5Yale UniversityWe consider a semiconductor quantum dot coupled to a Fermionicreservoir, and study the dynamics after a quantum quench induced bythe sudden creation of an exciton via optical absorption of an incidentphoton of definite frequency [1]. The subsequent emergence of corre-lations between the spin degrees of freedom of the dot and reservoir,ultimately leading to the Kondo effect, can be probed via a simple op-tical absorption experiment. The resulting lineshape is found to unveilthree very different dynamical regimes, corresponding to short, inter-mediate and long times after the initial excitation, which are in turndescribed by the three fixed points of the single-impurity AndersonHamiltonian. At low temperatures and just beyond the absorptionthreshold, the lineshape is dominated by a power-law singularity, withan exponent that is a universal function of magnetic field and gatevoltage. Analytical results obtained by fixed-point perturbation the-ory are in excellent agreement with numerical renormalization groupresults.

[1] arXiv:0907.3854v1 [cond-mat.str-el]

TT 4.6 Mon 11:45 H21Non-equilibrium Scaling Properties of a Double QuantumDot System: Comparison between Perturbative Renormal-ization Group and Flow Equation Approach — ∙VerenaKoerting1, Peter Fritsch2, and Stefan Kehrein2 — 1Niels BohrInstitute, Universitetsparken 5, DK-2100 København Ø, Denmark— 2Physics Department, Arnold Sommerfeld Center for TheoreticalPhysics and Center for NanoScience, Ludwig-Maximilians-Universität,Theresienstrasse 37, 80333 Munich, GermanySince the discovery of Kondo physics in quantum dots, its far-from-equilibrium properties have generated considerable theoretical interest.By now several new theoretical methods have analyzed the interest-ing interplay of non-equilibrium physics and correlation effects in thismodel.

In this talk the differences and commons between the flow equa-tion method out of equilibrium [1] and the frequency-dependent poorman’s scaling approach [2] will be presented. We will focus on thenon-equilibrium properties of a double quantum dot system, whichwill turn out to be a particularly suitable testing ground while beingexperimentally interesting in its own right.

[1] S. Kehrein, PRL 95, 056602 (2007).[2] A. Rosch, J. Paaske, J. Kroha, and P. Wölfle, PRL 90, 076804

(2003).

TT 4.7 Mon 12:00 H21Relaxation vs decoherence: Spin and current dynamics in theanisotropic Kondo model at finite bias and magnetic field —∙Mikhail Pletyukhov, Dirk Schuricht, and Herbert Schoeller— Institut für theoretische Physik A, RWTH Aachen, GermanyUsing a nonequilibrium renormalization group method we study thereal-time evolution of spin and current in the anisotropic Kondo model(both antiferromagnetic and ferromagnetic) at finite magnetic field ℎ0and bias voltage 𝑉 . We derive analytic expressions for all times inthe weak-coupling regime max{𝑉, ℎ0, 1/𝑡} ≫ 𝑇𝑐 (𝑇𝑐 = strong couplingscale). We find that all observables decay both with the spin relax-ation and decoherence rates Γ1/2. Various 𝑉 -dependent logarithmic,oscillatory, and power-law contributions are predicted. The low-energycutoff of logarithmic terms is generically identified by the difference oftransport decay rates. For small times 𝑡 ≪ max{𝑉, ℎ0}−1, we obtainuniversal dynamics for spin and current.

TT 4.8 Mon 12:15 H21Discretization artifacts in the TD-NRG — ∙Fabian Güttge andFrithjof B. Anders — Technische Universität Dortmund, Lehrstuhlfür Theoretische Physik II, 44221 Dortmund, GermanyOne approach to study the nonequilibrium dynamics of quantum im-purity systems is the time-dependent numerical renormalization-group(TD-NRG). Like the conventional NRG the TD-NRG relies on a log-arithmic discretization of the bath continuum by mapping the bathonto a Wilson chain. We demonstrate in the real-time dynamics ofa toy model, the resonant level model, that the group velocity of thewave propagation along the Wilson Chain is strongly position depen-


dent. The induced reflection of waves corrupt the calculation of thetime-dependent impurity occupation. The time period for which theoccupation number is calculated reliably can be extended by using ahybrid approach combining different discretizations of the bath con-tinuum.

TT 4.9 Mon 12:30 H21Nonequilibrium functional RG with frequency dependentvertex function – a study of the single impurity Andersonmodel — ∙Severin Jakobs1,2, Mikhail Pletyukhov1,2, and Her-bert Schoeller1,2 — 1Institut für Theoretische Physik A, RWTHAachen University, D-52056 Aachen, Germany — 2JARA – Funda-mentals of Future Information Technologies, GermanyWe investigate nonequilibrium properties of the single impurity An-derson model by means of the functional renormalization group (fRG)

within Keldysh formalism. We present how the level broadening Γ/2can be used as flow parameter for the fRG. This choice preserves im-portant aspects of the Fermi liquid behaviour that the model exhibitsin case of particle-hole symmetry. An approximation scheme for theKeldysh fRG is developed which accounts for the frequency depen-dence of the two-particle vertex in a way similar but not equivalentto a recently published approximation to the equilibrium MatsubarafRG. Our method turns out to be a rather flexible tool for the study ofweak to intermediate on-site interactions 𝑈 . 3Γ. In equilibrium wefind excellent agreement with NRG results for the linear conductanceat finite gate voltage, magnetic field, and temperature. In nonequi-librium, our results for the current agree well with TD-DMRG. Forthe nonlinear conductance as function of the bias voltage, we proposereliable results at finite magnetic field and finite temperature. Fur-thermore, we demonstrate the exponentially small scale of the Kondotemperature to appear in the second order derivative of the self-energy.

TT 5: CE: Low-dimensional Systems - Materials 1


Invited Talk TT 5.1 Mon 14:00 H18Field-Induced Berezinskii-Kosterlitz-Thouless Transition in a2d Spin-Dimer System — ∙Michael Lang1, Ulrich Tutsch1,Bernd Wolf1, Tonia Kretz2, Hans-Wolfram Lerner2,Matthias Wagner2, Stefan Wessel3, Tanusri Saha-Dasgupta4,Harald Jeschke5, and Roser Valenti5 — 1Phys. Institut, Univ.Frankfurt, SFB/TR49, D-60438 Frankfurt (M) — 2Inst. f. Anorg.Chemie, Univ. Frankfurt SFB/TR 49, D-60438 Frankfurt(M) — 3Inst.f. Theor. Phys. III, Univ. Stuttgart, D-70550 Stuttgart — 4S.N. BoseNational Centre f. Basic Science, 700098 Kolkata, India — 5Inst. f.Theor. Phys., Univ. Frankfurt, SFB/TR49, D-60438 Frankfurt(M)Weakly-coupled spin-1/2 dimer systems exposed to a sufficiently strongmagnetic field offer exciting possibilities for studying critical phenom-ena under well-controlled conditions [1]. A prominent example is theBose-Einstein condensation of magnetic triplet excitations in three di-mensionally (3D)-coupled systems. Here we report on a chemically-constructed multilayer bulk magnet composed of molecule-based pairsof spin S = 1/2 dimers, where, by the application of a magnetic field,a gas of magnetic excitations is formed. Based on magnetic suscep-tibility measurements combined with Density Functional Theory andQuantum Monte Carlo calculations, we conclude that these excitationshave a distinct 2D character and that the field-induced state, revealedat low temperatures of 39 mK, is a manifestation of the Berezinskii-Kosterlitz-Thouless topological order.

[1] T. Giamarchi et al., Nature Physics 4, 198 (2008).

TT 5.2 Mon 14:30 H18Exploring the doping dependence of the Mott transitionon X-ray irradiated crystals of 𝜅-(ET)2Cu[N(CN)]2]Cl —∙Ulrich Tutsch1, Ammar Naji1, Takahiko Sasaki2, and MichaelLang1 — 1Physikalisches Institut, Goethe-Universität Frankfurt (M),SFB/TRR49, D-60438 Frankfurt (M) — 2Institute for Materials Re-search, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi980-8577, JapanThe quasi two-dimensional organic charge-transfer salt 𝜅-(ET)2Cu[N(CN)2]Cl has a Mott-insulating ground state at ambientpressure, which can be transformed into a superconducting groundstate (𝑇𝑐 ≈ 13 K) by applying moderate pressures of ∼ 30MPa(300 bar). Our objective is to study how the first-order Mott-transitionline and its second-order critical end point change on doping the mate-rial away from half filling. We use X-ray irradiation in order to intro-duce charge carriers in this material [1] and take the shifts in the room-temperature resistivity as a measure of the amount of doping. We willpresent resistivity data for the temperature range 5K ≤ 𝑇 ≤ 60Kand for pressures up to 50MPa for a 𝜅-(ET)2Cu[N(CN)]2]Cl crystalat various doping levels and discuss the accompanied changes in thep-T -phase diagram.

[1] T. Sasaki et al., J. Phys. Soc. Jpn. 76, 123701 (2007)

TT 5.3 Mon 14:45 H18Finite size effects and magnetic order in the spin-1/2 hon-eycomb lattice compound InCu2/3V1/3O3 — ∙M. Yehia1, E.Vavilova1,2, U. Löw3, A. Möller4,5, T. Taetz5, R. Klingeler1,V. Kataev1, and B. Büchner1 — 1Leibniz Institute for Solid State

and Materials Research IFW Dresden, D-01171 Dresden, Germany —2Zavoisky Physical Technical Institute, Russian Academy of Sciences,420029 Kazan, Russia — 3Technische Universität Dortmund, Theo-retische Physik I, 44221 Dortmund, Germany — 4University of Hous-ton, Department of Chemistry and Texas Centre for SuperconductivityHouston, TX 77204, USA — 5Institut für Anorganische Chemie, Uni-versität zu Köln, 50939 Köln, GermanyA two dimensional spin honeycomb lattice on the basis ofInCu2/3V1/3O3 was studied by means of high field electron spin res-onance, nuclear magnetic resonance and magnetic susceptibility. Pre-vious structural studies suggest the occurrence of uncorrelated finitesize in-plane domains, which is expected to inhibit long range mag-netic order. Surprisingly, ESR data reveal the development of twocollinear AFM sublattices below ∼ 20 K whereas NMR results showthe presence of the staggered internal field. This is consistent with themagnetization data which implies a reorientation of the spin sublat-tices at ∼ 5.7 T. Quantum Monte-Carlo calculations of spin clustersof the coupled honeycomb spin planes indicate the development of thestaggered magnetization at a finite temperature. This may explain theoccurrence of the AFM state in InCu2/3V1/3O3 despite unfavorablefor magnetic order structural effects.

TT 5.4 Mon 15:00 H18Low temperature magnetic response of CePt surface alloy— ∙Annemarie Köhl, Christian Praetorius, Sebastian Brück,and Kai Fauth — Physikalisches Institut, Universität Würzburg, AmHubland, 97074 Würzburg, GermanyWe have used X-ray magnetic circular dichroism (XMCD) at Ceriumedges to determine the low temperature paramagnetic local momentresponse of a CePt intermetallic surface alloy. The ultrathin alloy filmis prepared in situ by evaporation of few atomic layers of Ce ontoPt(111) and subsequent annealing. LEED patterns of the resulting al-loy hint at a (1.85×1.85) superstructure. The temperature dependentsusceptibility is partially compatible with the response of the crystalfield split Ce 4f1 state with a crystal field splitting of ≈ 60 K. Below 20K our data are indicative of a magnetic phase transition at a criticaltemperature of ≈ 9 K. We discuss our results in view of the recentfindings of Kondo lattice physics in the related CePt5 surface alloy.

TT 5.5 Mon 15:15 H18Magnetism in Azurite Studied by Muon Spin Rotation —Mathias Kraken1, Josefin Engelke1, Stefan Süllow1, ∙JochenLitterst1, Anja Wolter2, Bernd Wolf3, Michael Lang3, ChrisBaines4, and Hubertus Luetkens4 — 1IPKM, Technische Univer-sität Braunschweig, Braunschweig, Germany — 2IFW , Dresden, Ger-many — 3Physikalisches Institut, Universität Frankfurt, Frankfurt amMain, Germany — 4LEM, PSI, Villigen, SwitzerlandThe natural mineral azurite Cu3(CO3)2(OH)2 represents a new typeof low-dimensional frustrated quantum spin system with a diamondspin chain as basis. From specific heat [1] there is evidence for a phasetransition at ca. 1.8 K which however is magnetically still ill-defined.Earlier muon spin rotation experiments [2] have indicated a magnetictransition yet no systematic study has been reported. We have per-


formed zero field and transverse field muon spin rotation experimentsat Paul Scherrer Institut Villigen (Switzerland) in the temperaturerange from 0.02 K to 6 K on polycrystalline powder and a single crys-tal. We could corroborate the appearance of magnetic order below 1.9K from spontaneous muon spin rotation with a frequency following amagnetization curve. The rotation profile is indicative of a modulatedspin structure. In addition we find a pre-cursor phase between 1.9 Kand about 3 K which we relate to the onset of magnetic correlationsbetween Cu monomers via dimers.

[1] H. Kikuchi, et al., Phys. Rev. Lett. 94 (2005) 227201.[2] H. Kikuchi, et al., Progress of Theoretical Physics Supplement.

159 (2005)1 2005

TT 5.6 Mon 15:30 H18Neutron scattering investigations of the magnetic proper-ties of azurite Cu3(CO3)2(OH)2 — ∙Kirrily Rule1, ClareGibson1,2, Manfred Reehuis1, Bachir Ouladiaff3, MatthiasGutmann4, Jens-Uwe Hoffmann1, Sebastian Gerischer1, AlanTennant1,2, Stefan Süllow5, and Michael Lang6 — 1Helmholtz-Zentrum Berlin, Berlin, Germany — 2Institut für Festkörperphysik,TU Berlin, Berlin, Germany — 3Institut Laue-Langevin, Grenoble,France — 4ISIS Facility, Rutherford Appleton Laboratory, Didcot UK— 5Institut für Physik der Kondensierten Materie, TU Braunschweig,Braunschweig, Germany — 6Physikalisches Institut, J.W. Goethe-Universität Frankfurt, GermanyAzurite, Cu3(CO3)2(OH)2, has been considered an ideal example of aone-dimensional (1D) diamond chain antiferromagnet. Early studies ofthis material imply the presence of an ordered antiferromagnetic phasebelow TN 1.9 K while magnetization measurements have revealed a1/3 magnetization plateau. Until now, no corroborating neutron scat-tering results have been published to confirm the ordered magneticmoment structure. In this talk, we will present neutron diffractiondata which reveal the presence of a commensurate magnetic order inazurite. The results of magnetic structural refinement from single crys-tal diffraction will also be discussed. Finally we will show some recentinelastic neutron scattering results which reveal new information aboutthe dynamics in this material.

15 min. break

TT 5.7 Mon 16:00 H18Consequence of the intra-chain dimer-monomer spin frustra-tion and the inter-chain dimer-monomer spin exchange inthe diamond-chain compound Azurite Cu3(CO3)2(OH)2 —∙R. K. Kremer1, J. Kang2, C. Lee2, and M.-H. Whangbo2 —1Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany —2Department of Chemistry, North Carolina State University, Raleigh,North Carolina 27695-8204The spin lattice appropriate for Azurite Cu3(CO3)2(OH)2 was de-termined by evaluating its spin exchange interactions on the basis offirst principles density functional calculations. It is found that Azuritecannot be described by an isolated diamond chain with no spin frus-tration, but by a two-dimensional spin lattice in which diamond chainswith spin frustration interact through the interchain spin exchange inthe ab-plane.[1] Our analysis indicates that the magnetic properties ofAzurite at low temperatures can be approximated by two independentcontributions, i.e., an isolated dimer and an effective uniform chaincontributions. This prediction was verified by analyzing the magneticsusceptibility and specific heat data of Azurite.

[1] J. Kang, et al. J. Phys. Cond. Matter 21 392201 (2009).

TT 5.8 Mon 16:15 H18Effective spin-chain model for azurite: derivation from ab-initio computations (exchanged with TT 5.9) — Ingo Opahle1,Hem C. Kandpal2, Niels Jackson1, Hena Das3, Tanusri Saha-Dasgupta3, Andreas Honecker4, ∙Harald O. Jeschke1, andRoser Valentí1 — 1Institut für Theoretische Physik, Goethe-Universität Frankfurt — 2IFW Dresden — 3Bose Institute, Kolkata,India — 4Institut für Theoretische Physik, Universität GöttingenThe observation of complex spin dynamics and a 1/3 magnetiza-tion plateau has revived the interest in the famous pigment azuriteCu3(CO3)2(OH)2. We revisit the question of the underlying micro-scopic Hamiltonian using a combination of first principles methods.As a guiding tool, we employ NMTO downfolding which yields therelative importance at low energies of the many possible hybridizationpaths between the copper centers. In order to obtain the corresponding

exchange coupling strengths we perform total energy calculations formany spin configurations and several supercells. We employ FPLAPWand FPLO basis sets with LDA+U functionals, and we investigate thedependence of the exchange couplings on the correlation strength U.We find that the low-energy Hamiltonian of azurite is a diamond chainwith a monomer-monomer coupling and some nonzero interchain cou-plings. We show that the neglect of interchain couplings effectivelyleads to a less symmetric diamond chain.

TT 5.9 Mon 16:30 H18Effective spin-chain model for azurite: comparison with ex-perimental results (exchanged with TT 5.8) — ∙AndreasHonecker1, Robert Peters1, Thomas Pruschke1, RoserValentí2, Harald Jeschke2, Ingo Opahle2, Hem Kandpal2,Tanusri Saha-Dasgupta3, Hena Das3, Johannes Richter4,Helge Rosner5, Oleg Janson5, Shijie Hu6, Xiaoqun Wang6,Bernd Wolf2, and Michael Lang2 — 1Institut für TheoretischePhysik, Georg-August-Universität Göttingen — 2Johann WolfgangGoethe-Universität Frankfurt am Main — 3Bose National Centre forBasic Sciences, Kolkata — 4Institut für Theoretische Physik, Otto-von-Guericke Universität Magdeburg — 5Max-Planck-Institut für Chemis-che Physik fester Stoffe, Dresden — 6Department of Physics, RenminUniversity of China, BeijingWe analyze a spin-1/2 Heisenberg model with a generalized diamondchain exchange geometry which has been derived from ab-initio com-putations for the mineral azurite Cu3(CO3)2(OH)2. Using numericalresults, we demonstrate that a consistent description can be obtainedfor various physical properties of azurite: (i) the low-temperature mag-netization curve, (ii) inelastic neutron scattering on the 1/3 magneti-zation plateau, (iii) nuclear magnetic resonance measurements on the1/3 magnetization plateau, and (iv) the magnetic susceptibility as wellas the specific heat. Our results resolve previous controversies on themodeling of azurite. Furthermore, we explain why a one-dimensionalmodel can be used in many situations although interchain exchange inazurite is actually non-negligible at a microscopic level.

TT 5.10 Mon 16:45 H18The low-temperature structure of azurite — ∙Clare Gibson1,2,Kirrily Rule1, Manfred Reehuis1, Bachir Ouladdiaf3,Alan Tennant1,2, Stefan Suellow4, and Michael Lang5 —1Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109Berlin, Germany — 2Institut für Festkörperphysik, TU Berlin, Berlin,Germany — 3Institut Laue-Langevin, Grenoble, France — 4Institutfür Physik der Kondensierten Materie, TU Braunschweig, Germany —5Phys. Inst., J.W. Goethe-Universität Frankfurt, GermanyThe low-dimensional quantum magnet and natural mineral azurite,Cu3(CO3)2(OH)2, is considered to be a model of the distorted dia-mond chain lattice. Recently, much study has been devoted to deter-mining the magnetic exchange parameters in this frustrated magneticmaterial. However, results have been contradictory [1,2,3] and the va-lidity of the one-dimensional model has been called into question [4].An accurate structural determination of azurite below the Nèel tem-perature may prove useful for the purpose of exchange parameter cal-culation in terms of precise knowledge of the magnetic superexchangepathways. We report on single crystal and powder neutron diffractiondata taken over the temperature range 200 mK - 5 K to determinethe low temperature lattice parameters and space group. Analysis re-veals strain in the material which coincides with the magnetic orderingtransition at 1.86 K.

[1] H. Kikuchi et al., Phys. Rev. Lett. 94, 227201 (2005)[2] K.C. Rule et al., Phys. Rev. Lett. 100, 117202 (2008)[3] G. Bo and G. Su, Phys. Rev. Lett. 97, 089701 (2006)[4] J. Kang et al., J. Phys.: Condens. Matter 21, 392201 (2009)

TT 5.11 Mon 17:00 H18Analyzing the complex spin-coupling structure in the single-molecule magnet Mn12wheel by inelastic neutron scattering— ∙Joscha Nehrkorn1, Oliver Waldmann1, Taketo Tagushi2,George Christou2, Thierry Strässle3, Philip L. W. Tregenna-Pigott3, and Hannu Mutka4 — 1Physikalisches Institut, Univer-sität Freiburg, 79104 Freiburg, Germany — 2Department of Chemistry,University of Florida, Gainesville, Florida 32611-7200, USA — 3LNS,ETH Zürich & Paul Scherrer Institut, 5232 Villigen PSI, Switzerland— 4Institut Laue-Langevin, 38042 Grenoble, FranceRecently the single-molecule magnet (SMM) [Mn12(O2CMe)14(R-mda)8], or Mn12wheel in short, has attracted interest because of itsunusual quantum tunneling transitions in the magnetization, which


were explained by describing it as a dimer of two magnetically coupledSMM subunits [1,2]. In order to analyze the underlying complicatedspin structure we performed inelastic neutron scattering experiments.They show that the model of two coupled SMMs has to be refused. Amicroscopic model was devised which reproduces the data and providesvalues for the exchange coupling and magnetic anisotropy parameters.By using basic principles for bipartite lattices the low-energy sectorcan be reduced again to that of two coupled subunits, which, how-ever, are substantially different than the two subunits in the originallyproposed dimer model. The new model resolves some controversies asregarding the magnetic tunneling transitions.

[1] C. Ramsey et al., Nature Physics 4, 277 (2008)[2] W. Wernsdorfer et al., Phys. Rev. Lett 101, 237204 (2008)

TT 5.12 Mon 17:15 H18Molecular Magnets Confined in the Nanocage of a Globu-lar Protein — ∙Peter Lemmens1,4, Dirk Wulferding1,4, DirkMenzel1, Tamoghna Mitra2, Achim Müller2, Rajib KumarMitra3, Pramod Kumar Verma3, and Samir Kumar Pal3 —1IPKM, TU-BS, Braunschweig — 2FC, Univ. Bielefeld — 3SNBC,Kolkata, India — 4IGSM, TU-BS, BraunschweigWe investigate the effect of confinement and energy transfer on thedynamics of a molecular magnet known as a model system to studyquantum coherence. The polyoxovanadate V15 is incorporated into aprotein cavity and the energy transfer probed by time resolved exper-iments. Work supported by DFG and B-IGSM.

TT 6: TR: Nanoelectronics II: Spintronics and Magnetotransport


TT 6.1 Mon 14:00 H19Transport through an interacting quantum dot tunnel cou-pled to a ferromagnet with time-dependent magnetisation —∙Nina Winkler1, Michele Governale2, and Jürgen König1 —1Theoretische Physik and CeNIDE · Universität Duisburg-Essen —2School of Chemical and Physical Sciences · Victoria University ofWellington · New ZealandWe study adiabatic charge and spin pumping in an interacting quan-tum dot connected to one normal and one ferromagnetic lead. Ingeneral, this setup can work as a quantum pump when only the direc-tion of the lead magnetisation is varied in time [1]. We focus on theadiabatic-pumping regime. To account for a time-dependent lead mag-netisation, we generalise a diagrammatic real-time approach for adia-batic pumping through quantum dots with ferromagnetic leads [2, 3].We perform a systematic expansion in both frequency and tunnel-coupling strength, treating the on-site Coulomb interaction on thequantum dot exactly. We investigate the adiabatic charge and spintransport through the structure when pumping by periodically chang-ing the direction of the lead magnetisation up to second order in thetunnel-coupling strength.[1] M.V. Costache et al. , Phys. Rev. Lett. 97, 216603 (2006).[2] J. Splettstoesser et al. , Phys. Rev. B 74, 085305 (2006).[3] J. Splettstoesser et al. , Phys. Rev. B 77, 195320 (2008).

TT 6.2 Mon 14:15 H19Kondo Effect in single wall carbon nanotubes with ferromag-netic contacts — ∙Markus Gaaß, Andreas Hüttel, DominikPreusche, Lorenz Herrmann, and Christoph Strunk — Univer-sität Regensburg, Institut für Experimentelle und Angewandte PhysikWe investigate the interplay of the Kondo effect and magnetic con-tacts in quantum dots formed by single wall carbon nanotubes. Theregular spin-1/2 Kondo effect appears when the coupling between theelectrodes and the quantum dot is high enough. If the leads are magne-tized the Kondo resonance is exchange split at zero magnetic field. Wefabricated single wall carbon nanotube transport devices with electriccontacts made of PdNi which show a sufficient interface transparencyto observe the Kondo effect. Without any applied field the Kondo res-onance is indeed split. In some Coulomb valleys the splitting can becompensated by an external magnetic field on the order of 1 Tesla, inothers it remains finite. In addition a fine structure in the spectra isobserved which indicates a more complex level structure than expectedfor armchair carbon nanotubes.

TT 6.3 Mon 14:30 H19A singlet - triplet T+ based qubit — ∙Hugo Ribeiro1, JasonPetta2, and Guido Burkard1 — 1Department of Physics, Univer-sity of Konstanz, D-78457, Konstanz, Germany — 2Department ofPhysics, Princeton University, Princeton, NJ 08544, USAWe theoretically model a nuclear-state preparation scheme that in-creases the coherence time of a two-spin qubit in a double quantumdot. The two-electron system is tuned repeatedly across a singlet-triplet level-anticrossing with alternating slow and rapid sweeps of anexternal bias voltage. Using a Landau-Zener-Stückelberg model, wefind that in addition to a small nuclear polarization that weakly af-fects the electron spin coherence, the slow sweeps are only partiallyadiabatic and lead to a weak nuclear spin measurement and a nuclear-

state narrowing which prolongs the electron spin coherence. This re-solves some open problems brought up by a recent experiment [1]. Wealso show that the electronic two-spin states singlet and triplet T+are promising candidates for the implementation of a qubit in GaAsdouble quantum dots (DQD). A coherent superposition of the two-spinstates is obtained by finite time Landau-Zener-Stückelberg interferom-etry and the single qubit rotations are performed by means of an ex-ternal magnetic field with a typical amplitude of about 100 mT, whilecoherent manipulation can be done within ∼ 1 ns. We also study thenuclear induced decoherence, mainly due to hyperfine contact couplingbetween the electronic and nuclear spins, and compute the decoherencetime 𝑇 *2 ∼ 10 ns.

[1] D. J. Reilly et al., Science 321, 817 (2008).

TT 6.4 Mon 14:45 H19Charge ratchet from spin flip: Space-time symmetry paradox— ∙Sergey Smirnov1, Dario Bercioux2, Milena Grifoni1, andKlaus Richter1 — 1Institut für Theoretische Physik, UniversitätRegensburg, D-93040 Regensburg, Germany — 2Freiburg Institute forAdvanced Studies (FRIAS) and Physikalisches Institut, UniversitätFreiburg, D-79104 Freiburg, GermanyTraditionally the charge ratchet effect is considered as a consequenceof either the spatial symmetry breaking engineered by asymmetric pe-riodic potentials, or time asymmetry of the driving fields. Here wedemonstrate that electrically and magnetically driven quantum dissi-pative systems with spin-orbit interactions represent an exception fromthis standard idea. In contrast to the so far well established belief, acharge ratchet effect appears when both the periodic potential anddriving are symmetric. We show that the source of this paradoxicalcharge ratchet mechanism is the coexistence of quantum dissipationwith the spin flip processes induced by spin-orbit interactions [1]. Thequeerness of the charge ratchet current consists in the fact that thiscurrent, in contrast to early predictions for systems without spin-orbitinteractions, appears even when only one energy band provides elec-trons for transport and no harmonic mixing is present in the drivingfields. Such purely spin-orbit charge currents are controlled by the gatevoltage tuning the strength of the spin-orbit coupling. This peculiarityof the effect is very attractive from the experimental point of view.

[1] S. Smirnov, D. Bercioux, M. Grifoni and K. Richter, Phys. Rev.B 80, 201310(R) (2009).

TT 6.5 Mon 15:00 H19Orbitally phase coherent spintronics — Cheryl Feuillet-Palma1,2, Thomas Delattre1,2, Pascal Morfin1,2, Jean-Marc Berroir1,2, Gwendal Feve1,2, Christian Glattli1,2,3,Bernard Placais1,2, Audrey Cottet1,2, and ∙Takis Kontos1,2— 1Laboratoire Pierre Aigrain, Ecole Normale Supérieure, 24, rueLhomond, 75231 Paris Cedex 05, France — 2CNRS UMR 8551, Labo-ratoire associé aux universités Pierre et Marie Curie et Denis Diderot,France — 3Service de physique de l’état Condensé, CEA, 91192The scattering imbalance between up and down spins at the interfacebetween a non-magnetic metal and a ferromagnetic metal is at theheart of the principle of the magnetic tunnel junctions or multilayerscelebrated in the field of spintronics. Although these devices use thequantum mechanical spin degree of freedom and electron tunneling,they do not exploit a crucial degree of freedom involved in quantummechanics: the phase of the electronic wave function. In most of the


devices studied so far, this aspect has not been developed owing to theclassical-like motion of charge carriers in the conductors used. In thiswork, we report on spin dependent transport measurements in carbonnanotubes based multi-terminal circuits. We observe a gate-controlledspin signal in non-local voltages and an anomalous conductance spinsignal, which reveal that both the orbital phase and the spin can beconserved along carbon nanotubes with multiple ferromagnetic con-tacts. This paves the way for spintronics devices exploiting both thesequantum mechanical degrees of freedom on the same footing.

TT 6.6 Mon 15:15 H19Bulk transport properties of two-dimensional HgTe nanos-tructures — ∙Elena G. Novik1, Patrik Recher2, Ewelina M.Hankiewicz2, and Björn Trauzettel2 — 1Physikalisches Insti-tut (EP3), University of Würzburg, 97074 Würzburg — 2Institut fürTheoretische Physik und Astrophysik, University of Würzburg, 97074WürzburgThe topologically non-trivial insulators realized in HgTe quantum wells(QWs) have recently attracted considerable attention because of theirunique property: the existence of the gap in the bulk and the gap-less edge states on the sample boundaries. Depending on the widthof the HgTe QW the structure can be a trivial insulator with nor-mal band structure when the QW width is smaller than 6.3 nm, ora topologically non-trivial insulator with inverted band structure forthicker QWs. Here we show that it is possible to distinguish the topo-logically trivial from non-trivial insulator states on the basis of bulktransport properties only. Using the effective four-band model [1],we have calculated the bulk conductance through the two-dimensionalmetal/HgTe insulator/metal structure. Whereas for the trivial insula-tor the conductance increases monotonically with decreasing distancebetween the electron reservoirs 𝐿, a non-monotonic behaviour of thebulk transport depending on 𝐿 has been found for the insulator inthe inverted regime. Interestingly, the bulk transport contribution caneven exceed the quantized conductance caused by edge state trans-port and should be taken into account for the interpretation of futureexperiments.

[1] B. A. Bernevig et al., Science, 314, 1757 (2006).

TT 6.7 Mon 15:30 H19Magneto-resistance of atomic-sized contacts of magnetic met-als — ∙Stefan Egle1, Hans-Fridtjof Pernau1, Cécile Bacca1,Magdalena Huefner2, and Elke Scheer1 — 1Department ofPhysics, University of Konstanz, Germany — 2Solid State PhysicsLaboratory, ETH Zurich, SwitzerlandIn this talk, a comprehensive study of the magneto resistance (MR)behavior of atomic-size contacts is given, where two macroscopic elec-trodes are connected to a central nanobridge. An atomic contactcan be adjusted by means of mechanically controllable break junction(MCBJ) technique. In order to separate the influence of the electrodesfrom the effect of the nanobridge itself, we employ different materialsystems. Namely, ferromagnetic cobalt electrodes are connected to anon-magnetic gold bridge and vice versa. The shape of the electrodescan be chosen symmetric or asymmetric and the nanobridge regioncan be suspended or non-suspended. Furthermore, we investigate theMR for different orientations of the magnetic field. The curves show avery rich behavior with magneto resistance ratios (MRR) up to 100%and more in the atomic contact regime, reaching up to a few 1000% inthe tunneling regime. For all geometries used, the MRR values are ofcomparable size. Moreover, we study the possible influence of the mi-cromagnetic order of the domains in the vicinity of the contact regionas well as ballistic MR, GMR, TMR, atomically enhanced anisotropicMR (AAMR) and magnetostriction. We conclude that the AAMR isthe most important contribution of the MR at large magnetic fields,while magnetostriction, TMR and GMR govern the low-field regime.

TT 6.8 Mon 15:45 H19Oscillatory crossover from two dimensional to three di-mensional topological insulators — ∙Chao-Xing Liu1, HaiJunZhang2, Binghai Yan3, Xiao-Liang Qi4, Thomas Frauenheim3,Xi Dai2, Zhong Fang2, and Shou-Cheng Zhang4 — 1EP3 andInstitute for Theoretical Physics and Astrophysics, University ofW�̈�rzburg, 97074 W�̈�rzburg, Germany — 2IOP, Chinese Academyof Sciences, Beijing 100190, China — 3Bremen Center for Compu-tational Materials Science, Universit�̈�t Bremen, 28359 Bremen, Ger-many — 4Department of Physics, Stanford University, Stanford, CA94305-4045Topological insulators (TIs) are new states of quantum matter with

the surface states protected by time-reversal symmetry, which can ex-ist both in two-dimensional (2D) system and three-dimensional (3D)system. In this work, we would like to investigate the crossover regimefrom 3D TIs to 2D TIs when the sample thickness is reduced. Basedon the four band effective model, we find that the crossover occurs inan oscillatory fashion as a function of the film thickness, alternatingbetween topologically trivial and non-trivial 2D behavior. A physicalpicture is provided to understand the origin of the oscillation. Further-more ab initio calculation is performed to study the realistic 𝐵𝑖2𝑆𝑒3and 𝐵𝑖2𝑇𝑒3 thin film and confirm the analytical results. These resultsnot only establisch the relation between the TIs with the different di-mensions, but also provide a new path to search for new TIs.

15 min. break

TT 6.9 Mon 16:15 H19Spin accumulation with spin-orbit interaction — ∙HenriSaarikoski1,2,3 and Gerrit E. W. Bauer1 — 1Kavli Instituteof Nanoscience, Delft University of Technology, 2628-CJ Delft, TheNetherlands — 2Mathematical Physics, Lund Institute of Technology,SE-22100 Lund, Sweden — 3Present address: University of Regens-burg, 93040 RegensburgSpin accumulation is a crucial but imprecise concept in spintronics.In metal-based spintronics it is characterized in terms of semiclassi-cal distribution functions. In semiconductors with a strong spin-orbitcoupling the spin accumulation is interpreted as a superposition of co-herent eigenstates. We show that both views can be reconciled bytaking into account the electron-electron interaction: a sufficientlystrong self-consistent exchange field reduces a spin accumulation toa chemical potential difference between the two spin bands even in thepresence of spin-orbit coupling. We demonstrate the idea on a cleantwo-dimensional electron gas (2DEG) by showing how the exchangefield protects a spin accumulation from dephasing and introduces aneasy-plane anisotropy. Spin can be injected either adiabatically, e.g.by a ferromagnetic contact with small electric bias, or diabatically, e.g.by pulsed optically induced excitation. We discuss spin-accumulationeigenstates that are accessible by adiabatic excitation as well as spinaccumulation dynamics of rapidly excited states. We illustrate thegeneral ideas at the hand of a 2DEGs with Rashba SOI, in which thedisorder-scattering lifetime broadening is much smaller than the spin-orbit splitting at the Fermi-level.

TT 6.10 Mon 16:30 H19Suppression of the Spin-Hall Effect by Magnetic Fields inNanostructures — ∙Dietrich Rothe and Ewelina M. Han-kiewicz — Institut für Theoretische Physik und Astrophysik,Würzburg, GermanyThe spin-Hall effect (SHE) is the generation of the transverse spin-imbalance in semiconductors with strong spin-orbit interactions in theconducting (”metallic”) regime. Although the SHE is caused by spin-orbit interaction and the time symmetry breaking perturbation shoulddestroy the phenomenon, the influence of magnetic field was not so fardiscussed in detail.

In this contribution, we will analyze the influence of magnetic fieldon the magnitude of spin signal in the H-shaped structures where thespin signal can be excited and detected electrically [1,2].

We will show, within the non-equilibrium Green function formalism,that the magnetic field destroys the spin signal effectively in perpen-dicular magnetic fields while the signal is much less affected in parallelfields where only the Zeeman part of the field is present. The connec-tion with experimental results will be discussed [1].

[1] C. Bruene, A. Roth , E. G. Novik, M. Koenig, H. Buhmann,E. M. Hankiewicz, W. Hanke, J. Sinova and L. W. Molenkamp,arXiv:0812.3768 (2008).

[2] E. M. Hankiewicz, L. W. Molenkamp, T. Jungwirth, and JairoSinova, Phys. Rev. B 70, 241301(R) (2004).

We acknowledge financial support by DFG HA 5893/1-1.

TT 6.11 Mon 16:45 H19Interference spin-blockade in symmetric nanojunctions —∙Andrea Donarini, Georg Begemann, and Milena Grifoni —Institut für Theoretische Physik, Universität RegensburgNanojunctions with a high degree of spatial symmtry, like molecularjunctions, but also specially designed multiple quantum dot structures,have a degenerate many body spectrum. The degeneracy is in fact pro-tected by symmetry. The interference between the degenerate states


causes a novel current blocking mechanism that allows, in presence ofpolarized leads, the all electric control of the spin and orbital degreeof freedom on the junction. We present here a general formalism togive necessary and sufficient conditions for interference blockade. Asan exa

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€¦ · LowTemperaturePhysicsDivision(TT) Overview Low Temperature Physics Division Fachverband Tiefe Temperaturen (TT) RudolfGross Walther-MeissnerInstitut BayerischeAkademiederWissensch